Ultra high molecular weight polyethylene (UHMWPE) is a type of polyolefin. It is made up of extremely long chains of polyethylene. It derives its strength largely from the length of each individual molecule (chain). Van der Waals bonds between the molecules are relatively weak for each atom of overlap between the molecules, but because the molecules are very long, large overlaps can exist, adding up to the ability to carry larger shear forces from molecule to molecule. Each chain is bonded to the others with so many Van der Waals bonds that the whole of the inter-molecule strength is high.
The polar groups present in most polymers easily bond with water. UHMWPE does not absorb water readily as polar groups are absent in it which makes its bonding with other polymers difficult. Therefore, skin does not interact with it strongly, making the UHMWPE fiber surface feel slippery. As UHMWPE does not contain chemical groups such as esters, amides or hydroxylic groups, that are susceptible to attack from aggressive agents, it is very resistant to water, moisture, most chemicals, UV radiation, and micro-organisms. UHMWPE are immune to aromatic stacking interactions caused in contact with aromatic solvents to which aromatic polymers are often susceptible.
The UHMWPE results in excellent properties such as a high abrasion resistance, high impact resistance and high melt viscosity. Hence it is used in applications where the lower molecular weight grades fail.
Existing Knowledge:
The synthesis of UHMWPE using conventional Zeigler Natta catalysts result in high entanglement of the chains because of the drastic reaction conditions used in such reactions. It is hence imperative that for achieving disentanglement of the polymer chains single site catalysts are needed which are highly active and exhibit living nature under milder process conditions.
The living nature of the catalyst systems is exemplified in the following patents of Fujita et al, Mitsui chemicals: U.S. Pat. No. 6,875,718; EP 0 874 005; PCT Int. Appl. WO 2001005231.
Geoffrey W Coates et al have also shown the living nature of such related catalyst systems vide U.S. Pat. Nos. 6,562,930; 6,787,624; 7,119,154 for the homo and copolymerization of propylene.
Sanjay Rastogi has also used such catalyst systems utilising its living nature to demonstrate that the UHMWPE produced was of the disentangled nature (EP 1 308 255 dated 2003). His recent US patent application 2010/0087929 demonstrates the usage of such disentangled UHMWPE for mouldable shaped parts with advantages for use in medical applications.
Teijin Aramid, N L, used such disentangled UHMWPE and developed suitable machinery to process same using solid state processing techniques followed by drawing into tapes and fibers as exemplified in the following patent applications. WO 2009/153318, WO 2010/003971, WO 2009/133060, WO 2009/109632, WO 2010/007062, WO 2006/136323, WO 2009/133060
WO2009127410A1 describes a method of preparation of a ultra high molecular weight polyethylene (hereinafter mentioned as “UHMWPE”) of molecular weight between 1000000 g/mol and 10000000 g/mol; by using a titanium based catalyst and a co-catalyst triethylaluminium or triisobutylaluminium which has a molar ratio of magnesium compound:titanium compound lower than 3:1.
EP874005B1 deals with olefin polymerization catalysts such as a transition metal complex and organoaluminum oxy-compound as a co-catalyst and processes for synthesizing polyethylene.
U.S. Pat. No. 5,986,029 deals with a process for synthesis of HDPE, LDPE, LLDPE and co-polymers of ethylene using a new catalyst transition metal complex like C5Me(CH2)2NMe2TiCl2 and its analogues and methyl alumoxane co-catalyst system.
US2003096927 deals with the synthesis of Polyethylene and its co-polymer using catalyst Bis—schiff base ligand with a cyclopentadienyl metal chloride where the metal is titanium or zirconium or hafmium and a co-catalyst methyl alumoxane.
JP2000063416 deals with the snythesis of polymers with a wide molecular weight distribution using a titanium catalyst and organoaluminumoxy compound as a co-catalyst.
CN1651472 deals with a catalyst of a IV B-family match of Schiff base containing oxyl, which has a catalytic activity for polymerising ethylene.
CN1850869 discolse a beta-ketimine vanadium alkene polymerization catalyst and the manufacture method and the application of ethylene polymerising, ethylene and norborene polymerizing, ethylene and alpha-alkene or norborene co-polymerization.
WO09091334 deals with the synthesis of self assembled catalyst which is a Li or Na salt of Schiff base made from (an aniline and di-aldehyde/di-ketone, tri-aldehyde/tri-ketone or tetrakis-aldehyde/tetrakis-ketone) with transition metal and a co-catalyst like organo-aluminium compound and its use for the synthesis of polyolefins of low molecular weight to ultra high molecular weight (3000 to 3000000).
U.S. Pat. No. 6,562,930 is directed to bis(salicylaldiminato)titanium complex catalysts, and highly syndiotactic polypropylene makable therewith by a chain-end control mechanism, and block copolymers containing the syndiotactic polypropylene and poly(ethylene-co-propylene) and/or poly(alpha-olefin-co-propylene), as well as to living olefin polymers and to olefin terminated to oligomers and polymers and to methods of making syndiotactic polypropylene, block copolymers and olefin-terminated oligomers and polymers from propylene.
EP1669376 relates to a dual catalyst system on the same support which is a phosphinimine ligand supported catalyst and aluminium complex with a hindered phenol as co-catalyst suitable for the polymerization of bimodal polyolefins (e.g. polyethylene) having reverse or partial reverse comonomer incorporation.
US2003060584 is directed to a process for synthesis of syndiotactic polypropylene and its block co-polymers using bis(salicylaldiminato)titanium complex and methyl alumoxane.
The state of the art discloses processes for the manufacture of polyolefins including UHMWPE. However, the UHMWPE suffers from the drawback that it can be processed only at high temperatures. Because of the high molecular weight and high melt viscosity of UHMWPE, it requires specialized processing methods like compression moulding and ram extrusion. Furthermore, processing of UHMWPE for making fibers for using in bullet proof vests involves high temperature gel spinning of polymer solutions which are energy intensive. There is a need to synthesize disentangled form of UHMWPE which can be processed at lower temperatures and can be used for a variety of purposes. The existing methods for synthesis of DUHMWPE makes use of cryogenic units for maintaining zero to sub-zero temperatures to carry out the reaction. Therefore, the synthesis of a DUHMWPE in a simple and efficient manner is a challenge. Thus, there remains a need for developing an efficient, economically viable and industrially feasible process for the synthesis of DUHMWPE.